1. Field of the Invention
The present invention relates to an ion source for an ion attachment mass spectrometry apparatus, and more particularly, to an ion source used for an ion attachment mass spectrometry apparatus which attaches metal ions emitted from an emitter to a detected gas to ionize it and analyze the mass of the detected gas.
2. Description of the Related Art
In mass analysis of gas molecules, it has been necessary to give a positive or negative charge to the gas molecules in order to make use of the fact that the motion of charged particles in an electromagnetic field differs depending on the ratio between the charge and the mass. As methods for ionizing the gas molecules, there are the electron impact ionization method, the chemical ionization method, the atmospheric pressure ionization method, and the ion attachment ionization method, etc. Among these, the ion attachment ionization method enables ionization without dissociation (splitting) of the gas molecules including weak bonds since the excess energy arising in the process of ionization of a detected gas is extremely small. Therefore, in a mass spectrometry apparatus, it is possible to measure the correct molecular weight of a detected gas from the molecular ion peaks according to the ion attachment ionization method. This is effective for mass analysis of easily dissociating organic samples.
The ion attachment ionization method uses the phenomenon that when a metal oxide (insulator) is heated and metal atoms contained are emitted as ions, these metal ions gently deposit at locations where the charges of the gas molecules concentrate. In particular, if an oxide containing an alkali metal is heated, it is known that positive charge metal ions are easily emitted from the surface thereof. Attaching the alkali metal ions to other gas molecules to ionize them has been reported in Analytical Chemistry, vol. 48, no. 6, p. 825 (1976) as the Hodges system, in Analytical Chemistry, vol. 56, no. 3, p. 396 (1984) as the Bombick system, and in Journal of Applied Physics, vol. 82, no. 5, p. 2056 (1997) as the Fujii system.
Next, an explanation will be given of a conventional ion source used in a mass spectrometry apparatus employing the ion attachment ionization method with reference to FIG. 10 to FIG. 12. FIG. 10 is a schematic view of the configuration of the ion source, FIG. 11 is an enlarged sectional view of the emitter, and FIG. 12 is an equivalent circuit diagram of the emitter.
As shown in FIG. 10, the ion source employing the ion attachment ionization method is comprised of a conductive casing (container) 101 forming an ion attachment region inside it and having one end completely open, an aperture 102 attached to the right open end of the casing 101, a voltage-impressed portion 103 passing through a part of the casing 101 while electrically insulated from the same, a spherical emitter 104 comprised of a metal oxide attached to a suitable position of the voltage-impressed portion 103, and a gas inlet 105 for introducing a detected gas and other gases into the ion attachment region. The aperture 102 has an opening 106 for passing the ionized detected gas. By providing an insulator 107 at the connecting portion with the open end of the casing 101, it is electrically insulated from the casing 101. Further, the voltage-impressed portion 103 is connected to a heating power source 108 and a bias power source 109.
The spherical emitter 104, as shown in FIG. 11, is fixed by sintering for example to a wire-shaped voltage-impressed portion 103. The diameter of the emitter 104 is about 2 to 3 mm, for example. The portion of the voltage-impressed portion 103 in contact with the emitter 104 will be particularly referred to as a reference-voltage-impressed portion 103a. The emitter 104 is a mixture of an alumina silicate comprised of Al2O3 or SiO2 and an oxide (compound) containing Li, that is, Li2O, when the metal ions to be emitted from the emitter are Li+ ions. These are all oxides, so form insulators overall. The specific resistance is also at least 1012 xcexa9xc2x7m. At least the reference-voltage-impressed portion is a wire-shaped structure of a high melting point metal such as Ir (iridium) or W (tungsten). In the reference-voltage-impressed portion, Joule heat is generated by the flow of current.
In the above ion source, the aperture 102 is held at the ground voltage and a mixed gas of the detected gas and another gas is introduced through the gas inlet 105 into the ion attachment region evacuated to a vacuum state. The inside is evacuated to a reduced pressure atmosphere of about 100 Pa. The other gas is a gas such as N2 to which metal ions do not easily attach. This is introduced so as to rob the excess energy produced when the metal ions are attached to the detected gas. The voltage-impressed portion 103 is supplied with a bias voltage by the bias voltage source 109 so that the reference-voltage-impressed portion 103a becomes 10V, for example. Further, the heat source 108 lets a current flow at the reference-voltage-impressed portion 103a and thereby the emitter 104 is heated to about 600xc2x0 C. Due to the above operation, metal ions (Li+) are generated on the surface of the emitter 104. These metal ions are attracted by the electric field formed in the space 110 between the emitter 104 and the ground potential aperture 102, dissociated (emitted) from the surface of the emitter, and transported in the direction of the aperture 102. Next, the metal ions attach to the detected gas introduced into the ion source so as to ionize the detected gas.
In the above-described conventional ion source, the emitter is produced from an insulating metal oxide, so there was the problem that a potential difference between the reference-voltage-impressed portion 103a and the ion emission point on the surface of the emitter 104 cyclically changes. Since the emitter is an insulator, a large electrical resistor is interposed between the reference-voltage-impressed portion and the ion emission point. The above problem is caused by the fact that there is a voltage drop at the insulator.
FIG. 12 shows the portion between the reference-voltage-impressed portion and the ion emission point by an equivalent circuit. An electrical resistor 112 is interposed between the reference-voltage-impressed portion 103a and the ion emission point 111. In FIG. 12, when ions are emitted as shown by the arrows 113 from the emitter 104, a current flows through the electrical resistor 112 having a large resistance value. A voltage drop occurs here and the potential at the ion emission point 111 falls. The relation of the voltage drop is expressed as
Vb=Vaxe2x88x92Ixc2x7Rxe2x80x83xe2x80x83(1)
where the potential of the reference-voltage-impressed portion 103a is Va, the resistance of the emitter 104 is R, the current flowing through the emitter 104 is I, and the potential of the ion emission point 111 is Vb. Based on this relation, if the potential Vb at the ion emission point 111 falls, the electric field between the ion emission point 111 and the aperture 102 becomes weak, the amount of ion emission falls, and the current (I) flowing through the emitter 104 falls. If the current (I) falls, the voltage drop becomes smaller and the potential of Vb rises, so the amount of ion emission again increases. In this way, the process of xe2x80x9cVb dropxe2x86x92I fallxe2x86x92Vb risexe2x86x92I risexe2x86x92Vb dropxe2x80x9d is repeated and an unstable cyclical change of the amount of ion emission and the electric field continues. In the ion attachment mass spectrometry apparatus, to accurately detect the number of molecules of the ionized detected gas as an electrical signal, that is, to correctly analyze the mass, the amount of ion emission has to be stable. Therefore, if such a cyclic state of change arises, it is not possible to correctly analyze the mass of the detected gas.
As a means for solving the above problems, if it is desirable to merely make the ratio of the change in potential at the ion emission point 111 smaller, it will be considered to increase the bias voltage applied to the reference-voltage-impressed portion 103a. However, if the bias voltage is increased, the energy of the ions emitted from the surface of the emitter also becomes higher. As a result, the energy of the emitted ions striking the detected gas becomes higher and the other problem of dissociation of the detected gas arises. In the ion attachment ionization method, it is necessary that the metal ions be attached to the detected gas gently by a low energy. Therefore, it is not possible to increase the bias voltage applied to the reference-voltage-impressed portion 103a. 
An object of the present invention is to provide an ion source of an ion attachment mass spectrometry apparatus designed to suppress the occurrence of fluctuations in the potential difference between the ion emitter and the reference-voltage-impressed portion, stabilize the amount of ion emission, and enable high accuracy mass analysis.
The ion source of the ion attachment mass spectrometry apparatus according to the present invention is configured as follows to achieve the above object.
The ion source of the ion attachment mass spectrometry apparatus according to the present invention has an emitter containing a metal and a voltage-impressed portion impressing a bias voltage to the emitter. It heats the emitter to emit positive charge metal ions and attach the metal ions to the detected gas to ionize the gas. In this ion emission mechanism, by changing the material of the above emitter, the electrical resistance between the ion emission point of the emitter and the reference-voltage-impressed portion of the voltage-impressed portion is reduced.
In the above configuration, preferably, the material of the emitter is made a composite material of a compound containing a metal and a conductor. This composite material is a composite formed using either of the compound and the conductor as a base material and adding the other to it. Further, in the above configuration, preferably part of the material of the voltage-impressed portion is changed and that portion is formed as the emitter. The electrical resistance between the ion emission point and the reference-voltage-impressed portion is preferably not more than 1010xcexa9. Further, the above conductor is preferably one of gold, carbon, iridium, platinum, tantalum, rhenium, molybdenum, and composites of the same.
Normally, the electrical resistance is proportional to the specific resistance. When the specific resistance is the same, the resistance is proportional to the length of the resistor and is inversely proportional to the sectional area. In the present invention, the material of the emitter for emitting the metal ions is made a composite material of a metal oxide and a conductor. Due to this, the specific resistance of the emitter falls and the electrical resistance between the reference-voltage-impressed portion and ion emission point is reduced. The composite material used in the present invention functions to reduce the specific resistance due to the functions arising from the combination of materials. Due to this, the electrical conductivity rises and it becomes possible to eliminate cyclic fluctuations at the time of ion emission.
The ion source of the ion attachment mass spectrometry apparatus having another configuration is an ion source having the same underlying configuration as the above. By shortening the distance between the reference-voltage-impressed portion of the voltage-impressed portion and the ion emission point of the emitter, the electrical resistance between the ion emission point and the reference-voltage-impressed portion is reduced to the value being not more than 1010xcexa9.
In the above configuration, preferably, a thin film emitter is formed on the surface of the reference-voltage-impressed portion, or on the surface of the reference-voltage-impressed portion of a flat plate shape so as to shorten the distance between the ion emission point and the reference-voltage-impressed portion. Further, the above reference-voltage-impressed portion may be formed into a coil or hair pin shape.
In the above configuration, further, preferably the surface of the emitter is covered with a mesh like metal wire electrically conductive with the reference-voltage-impressed portion in a state of contact or else part or all of the surface of the emitter is covered with a conductive thin film having fine holes electrically conductive with the reference-voltage-impressed portion, whereby the electrical resistance between the ion emission point and the reference-voltage-impressed portion can be reduced and the distance between the two can be substantially reduced.
Here, the xe2x80x9cdistance between the two can be substantially reducedxe2x80x9d means that, while the distance between the two is not physically shortened, a similar action and effect are caused as a result of the reduction of the electrical resistance.
In the second aspect of the present invention, a structure shortening the distance between the reference-voltage-impressed portion and the ion emission point was adopted for the emitter so as to reduce the electrical resistance between the reference-voltage-impressed portion and the ion emission point. This drop in the electrical resistance can be achieved by a structure increasing the sectional area of the emitter in a direction perpendicular to the flow of the current or a structure adopting the above configuration.
The above-mentioned configurations, as the ion source able to be used in the ion attachment mass spectrometry apparatus, can obtain stable signals without causing a change in the potential difference between the ion emission point and the reference-voltage-impressed portion.
Note that as the metal ions, preferably use is made of any of Li+, K+, Na+, Rb+, Cs+, Al+, Ga+, and In+.